Low profile system for joining optical fiber waveguides

ABSTRACT

A compact, low profile splicing system for joining optical fibers produces durable, low transmission loss fusion splices. The system employs active optical techniques such as profile alignment or local injection and detection to achieve optimized alignment of the fibers prior to fusion. Light injected into one fiber is propagated across the interface to a second fiber. A detector senses the intensity of the injected light in the second fiber. After the relative position of the fibers is manipulated to maximize the transmitted intensity, the fibers are fusion spliced using an electric arc discharge. The accurate alignment achievable using the local injection and detection system to drive adaptive fiber positioning affords a method for reliably producing low loss splices. The present system is compact and low in profile, making it operable in cramped quarters with limited clearance to adjacent equipment and structures and with only a minimal amount of free fiber slack available. Simplicity of design and operation make the system rugged and enable accurate alignment and low loss fusion of fibers under adverse working conditions.

This application claims the benefit of application No. 60/456,915, filedMar. 24, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an apparatus for joining optical fiberwaveguides; and more particularly, to a low profile system thatadaptively positions the fibers being joined prior to fusion splicing,so that the transmission loss of the joined fiber is minimized.

2. Description of the Prior Art

Transmission of data by optical fiber waveguides, also called fiberoptics or optical fibers, has become ubiquitous in thetelecommunications and computer industries. Digital information in anelectronic system is converted into a series of pulses of lightgenerated by lasers or light emitting diodes (LED's), which are injectedinto long fibers of glass or polymeric materials. The fibers are capableof propagating the light with extremely low losses and acceptably lowdispersion, whereby information embodied in the modulation pattern maybe conveyed. The light that emerges from the other end of the fiber canbe detected and reconverted into electronic signals that faithfullyreproduce the original signal.

Fiber optic communication has a number of advantages over traditionaltransmission means such as hard-wired coaxial and twisted pair cable andlower frequency electromagnetic broadcasting such as radio andmicrowave. Foremost is the much larger bandwidth available. In addition,existing infrastructure such as cable ducts, utility poles, and the likepresently used by telecommunications companies can be upgraded withrelatively little disruption and moderate cost by substituting opticalfiber cable for existing copper wire. Thus, dramatic increases inbandwidth needed to accommodate the needs of an information-based,Internet-driven society and commerce can be obtained with comparativelylittle disruption.

Fiber optic communications have additional advantages for certainspecialized requirements. Fiber optic connections are far lessvulnerable to electromagnetic disruptions and nuclear radiation, whetherof natural origin or the result of the use of certain military weapons.Fiber optics are now widely used in aerospace and shipboard applicationsfor many of these reasons.

Implementation of fiber optic systems requires both the equipment foractual transmission and processing of the data, and the equipment neededto install and maintain the fiber optic system and its infrastructure.The transmission and processing equipment, such as the fiber itself andthe corresponding components needed to generate, detect, and processoptically-borne information, have been developed to an ever increasinglevel of sophistication. While certain systems for joining and splicingfiber optic cables have been developed, there remains a need in the artfor improved equipment and methods for splicing that are reliable,economical, and which result in minimal loss of signal integrity andstrength. Such systems, equipment, and methods are essential if the fullinherent advantages of optical transmission are to be more widelyimplemented.

The need for improved methods is especially acute for field installationand repair, which are frequently carried out under adverse conditions.Among the most significant needs is for effective means of splicingfiber optic cables both during initial installation and when repairs ormodifications are needed. In the telecommunications industry, repairsfrequently must be made to overhead lines by a technician operating froma ladder, lift bucket, sometimes during darkness and with adverseweather conditions such as precipitation, cold, and wind. Other repairsmust be made in cramped conditions in underground vaults and cablelockers.

Fiber optic communication systems are also commonly used for processcontrol, data, and voice communications in industrial and manufacturingfacilities. In these venues, the immunity of optical systems toelectronic and electromagnetically-induced noise and the elimination ofelectrical hazards are particularly beneficial. Cables in theselocations are often routed through tight quarters, some in hazardouslocations, making access for repair difficult. Communication systems onships and in airplanes and spacecraft likewise advantageously employfiber optic transmission; cable routing and access are often comparablyproblematic in these applications. In most of the aforementionedsituations repair is further hampered because of the limited length ofslack in the fiber that may be accessible for the technician tomanipulate into a splicing device. The need for a system usable formaking emergency repairs on fiber optic systems aboard militaryaircraft, ships, and submarines under operational or battle conditionsis especially acute.

Together, these considerations call for splicing systems that arecompact, portable, and able to be operated rapidly and reliably underadverse working conditions and with minimal slack cable. Moreover, it isdesired that such a splicing system be capable of joining two fibers ina way that (i) causes minimal disruption or discontinuity in the opticaltransmission, (ii) does not adversely increase the diameter and volumeof the cable, and (iii) has a durability as close as possible to that ofan original fiber. Systems are also desired that are simple and reliableenough to be used by technicians who lack extensive training. Thereremains an urgent need for optical splicing systems that satisfy theserequirements.

Optical fiber waveguides in common use share a number of structuralfeatures. The waveguide almost invariably comprises a thin, elongatedfiber core responsible for conducting the light and at least oneadditional layer. Most often the fiber core is highly pure glasssurrounded by a first and intimately-bonded layer termed a cladding andan outer layer called a buffer. The cladding, usually also glass, has anindex of refraction lower than that of the core to insure that light isconstrained for transmission within the core by total internalreflection. Typically the buffer is composed of plastic or polymer andserves to protect the inner layers mechanically and to prevent attack bymoisture or other substances present in the fiber's environment.Commonly a plurality of individual fibers (in some cases as many as athousand) constructed in this fashion are bundled together and enclosedin a protective jacket to form a cable.

Commonly used fibers may further be classified as multimode or singlemode. Multimode fibers typically comprise cores having diameters of50-62.5 μm but in some cases up to 100 μm. Single mode fibers generallyhave a much smaller core that may be 9 μm or less in diameter. Theglass-cladding diameter is most commonly 125 μm but sometimes is 140 μm(with a 100 μm core). The exterior diameter is largely a function of thebuffer coating, with 250 μm most common, although some fiber coatingsmay be as much as 900 μm in diameter. Alignment of fibers is a crucialpart of the preparation for any splicing operation, but is especiallychallenging for single mode fibers that have small core diameter. Inorder to produce a high quality, low-loss splice, the two opposing endsto be joined must be aligned laterally to within a small fraction of thecore diameter. Of course, the smaller the fiber diameter, the smallerthe allowed deviation from perfect abutting alignment that may betolerated.

Most fiber optic data transmission systems transmit information usingelectromagnetic radiation in the infrared band, including wavelengthssuch as 850 nm for multimode fibers and 1310 and 1550 nm for single modefibers. The nomenclature “light” is invariably employed for thisradiation, even though the cited wavelengths fall outside the rangevisible to humans.

Two general approaches for splicing optical fibers are in widespreaduse, viz. mechanical and fusion splicing. Mechanical splicing isaccomplished by securing the ends of two fibers in intimate proximitywith an aligning and holding structure. Often the fibers are insertedinto the opposing ends of a precision ferrule, capillary tube, orcomparable alignment structure. The fibers are then secured mechanicallyby crimping, clamping, or similar fastening. An adhesive is alsocommonly used. In some cases a transparent material such as a gel havingan index of refraction similar to that of the fiber cores is used tobridge the gap between the fibers to minimize reflection lossesassociated with the splice. Mechanical splicing is conceptually simple,and minimal apparatus is required to effect splicing. However, even inthe best case, a mechanical splice has relatively high and undesirableinsertion loss, typically 0.20 dB. In addition, mechanical splices aregenerally weaker than the underlying fiber and are notoriouslyvulnerable to degradation of the optical quality of the splice overtime, especially under adverse environmental conditions such as varyingtemperatures and high humidity. Mechanical splices are generallyregarded as being temporary expedients at best and are not useful forhigh bandwidth systems or permanent joints.

Fusion splicing entails the welding of the two fiber ends to each other.That is, the ends are softened and brought into intimate contact. Thesoftening is typically induced by a small electric arc struck betweenminiature pointed electrodes mounted in opposition and substantiallyperpendicular to the common axis of the fibers. Upon cooling, a strong,low-loss joint is formed. When properly carried out, fusion splicesexhibit very low losses along with high stability and durabilityrivaling those of the uncut fiber. Mechanical protection is oftenprovided by a heat-shrinkable tube applied over the completed joint. Thetube replaces the buffer coating that generally must be removed prior tosplicing. In many cases the heat-shrinkable tube is reinforced byincorporation therein of a length of metallic wire for stiffness.

One essential requirement for a low insertion loss splice is carefulpreparation and precise alignment of the ends of the fibers beingjoined. The axes of the fibers must be collinear within about 0.1 degreeand aligned laterally within a small fraction of the core diameter toachieve the desired loss of less than about 0.03 dB. This requiredprecision of alignment presents a substantial technical challenge,especially with single-mode fibers having cores approximately 9 μmdiameter. Three general approaches have been proposed in the prior art.The simplest expedient is the use of mechanical fixturing, such as thealignment ferrules described above and other forms of pre-alignedV-grooves and the like. These purely mechanical approaches do notreliably produce splices that maintain less than 0.10 dB loss and so areill suited for the demands of advanced, high-bandwidth communicationssystems. More sophisticated approaches employ some form of opticallyassisted fiber positioning. One such method is termed a profilealignment system (PAS). In this approach, the splicing apparatusincorporates an optical system that acquires images of the two fiberstaken in two lateral directions, allowing the fibers to be positioned intwo directions orthogonal to the mutual fiber axes. PAS systems mayincorporate either manual positioning or may employ computerized imageprocessing to optimize the alignment. However, the diffraction limit andpixel size of available electro-optic detectors restricts the precisionachievable with PAS, even in systems based on visible light withwavelengths of about 400-700 nm. This particularly compromises theeffectiveness of PAS in aligning small diameter, single mode fibers.

Still more advanced positioning methods have been proposed that employmeasurement of actual light transmission between the fibers beingjoined. The positioning of the fibers is adaptively adjusted to maximizelight transmission prior to the fusion operation. It is found that undercarefully controlled laboratory conditions this approach may permitalignment better than that achievable with PAS systems.

However, the methods and apparatus for carrying out splicing aidedeither by the PAS or by transmission-based alignment techniques haveheretofore not been well suited for use outside the laboratory or othersimilarly controlled workplace. The required equipment lacks theflexibility, versatility, and ruggedness needed for field use. Moreover,present equipment is cumbersome and not operable in the confined spacesfrequently encountered during field service.

Notwithstanding numerous advances in the field of fiber optic joining,there remains a need in the art for an economical, efficient process forforming low-loss, durable, and reliable splices in fiber optic cables.Also needed is portable splicing equipment that can be operated bytechnicians without extensive training to accurately and efficientlyjoin fiber optic cables in tightly confined spaces and under adverseenvironmental conditions.

SUMMARY OF THE INVENTION

The present invention provides a low profile system for joining opticalfibers by fusion splicing. The system is preferably modular and lowprofile, enabling it to be used to form low transmission loss splicesunder difficult field conditions. The loss of spliced fibers ispreferably minimized by use of automatically driven, active opticalsystems for adaptively aligning the cores of the two fibers prior tofusion. Such systems in some embodiments employ a profile alignment(PAS) system that employs a compact, imaging optical system incorporatedin a fusion splicing head of the system. More preferably, embodiments ofthe system further incorporate a low profile local injection anddetection (LID) system in carrying out the alignment. In the LIDtechnique, optimal alignment is signaled by maximization of thetransmission of light across the interface between the fibers. The LIDsystem further allows the transmission loss of the spliced fiber to beaccurately inferred.

In one aspect of the invention there is provided a low profile splicersystem for joining a first optical fiber and a second optical fiberalong a common fiber axis by fusion splicing. The system comprises a lowprofile fusion splicing head, a user interface, and electronic controlcircuitry. The splicing head employs a low profile fusion splicing headincluding a low profile fusion splicing stage having an electric arcwelding system; a clamping and fiber position adjustment systemcomprising holding means for holding the fibers substantially in ahorizontal plane and motion means for moving the fibers in threeorthogonal dimensions into coaxial, abutting alignment; and an imagingoptical system having a fiber imaging illuminator and a fiber imagedetector. The imaging optical system is adapted to acquire opticalimages of the fibers in a first imaging direction and a second imagingdirection, the imaging directions being non-coincident. The userinterface has an output display and user input controls for activatingthe splicing system. The electronic control circuitry comprises imagingelectronics that receive the output of the fiber image detector andproduce a display signal feeding the output display and fusion controlelectronics operably connected to activate the electric arc weldingsystem and supply high voltage thereto.

In an aspect of the invention, the imaging optical system employs afiber imaging illuminator comprising a first light source for the firstimaging direction and a second light source for the second imagingdirection; and a single image detector comprising a CMOS electro-opticaldevice. Preferably the optical system has a compact, folded optical pathto minimize the profile of the splicing stage and the splicing head.Light from the first source traverses a first optical path and lightfrom the second source traverses a second optical path, each of thepaths being multiply folded. The imaging optical system comprisesoptical elements located above and below the horizontal plane and thefirst and second optical paths lie in a plane perpendicular to thecommon fiber axis.

In some embodiments the system advantageously incorporates a profilealignment (PAS) system in communication with the fiber image detectorand the motion means, and the PAS system is adapted to automaticallycommand the motion means to bring the fibers into alignment prior to thefusion operation.

It is more preferred that the system employ a local injection anddetection system in carrying out fiber alignment. In an embodiment of aLID-based system, the splicing head of the system includes a local lightinjector and a detector that provides an electronic intensity signalindicative of the fraction of the injected light propagated across theinterface between the fibers; and the electronic control circuitrycomprises: (i) a driver energizing the light injector, (ii) measurementelectronics connected to the light detector receiving and processing theelectronic intensity signal to provide a measured intensity signal, and(iii) a servo system operative to drive the motion means to maximize themeasured intensity signal, whereby the relative position of the fibersis optimized prior to fusion.

The invention further provides a method for joining the fibers toproduce a splice having low transmission loss. The method comprises: (i)providing a low profile fusion splicing system such as theaforementioned LID-based system; (ii) preparing the fibers by removingany coatings such as buffer or cladding layers thereon and cleaving theends of the fibers to form a mating end on each; (iii) arranging thefibers in the splicing system's holding means with their ends in facingrelationship; (iv) imaging the fibers prior to joining; (v) positioningthe optical fibers into coaxial, abutting alignment; and (vi) fusing thefibers by electric arc welding. The optimization of fiber alignment ofis preferably carried out using either a PAS or a LID system.

The present fiber splicing system is modular, compact, and low profile.By “low profile” is meant a system having a small extent in the verticaldirection, i.e. the direction perpendicular to the plane in which thefiber path is located. Preferably, the vertical extent of the splicingsystem does not substantially increase as a result of the opening orclosing of the components that must be carried out to situate the fibersfor splicing with the splicing system it its operating location. That isto say, the vertical extent increases by at most about 2 mm as a resultof opening the various clamping and holding components. As a result ofthe advantageous configuration and operation of the present apparatus,fiber splicing can be carried out under adverse environmental conditionsand in cramped quarters. For example, the present invention isadvantageously employed in installing, repairing, and maintainingcommercial telecommunications cables, which often require a servicetechnician to operate from a lift truck or in underground vaults orcable lockers, often in adverse weather and under poor lighting. Thesystem is also useful for making emergency repairs of fiber opticsystems aboard military aircraft, ships, and submarines under theespecially acute challenges of operational or battle conditions.

The present system is easily modularized, with the fusion splicing headbearing only those components that directly impinge on the fibers beingjoined, with the associated control electronics, user interface andcontrols, and power supplies being connected but remotely located. Insome embodiments these additional components are mounted on a belt orvest worn by an operator or otherwise conveniently disposed forportability.

Generally stated, the present system employs active optical techniquesfor aligning the fibers prior to fusion splicing. In one aspect of theinvention, the system carries out this positioning using a profilealignment system (PAS). Suitable processing using a microprocessor orsimilar circuitry in the electronic control system infers the relativepositions in three dimensions of the two fibers from images thereofacquired using the imaging optical system. Suitable electronic commandsare issued to the motion means to bring the fibers into collinearalignment. Preferably the PAS system operates iteratively to effect themost precise alignment obtainable within the resolution of the opticalsystem.

A higher precision of alignment is generally attainable usingembodiments incorporating a local injection/detection system comprisinga light injector and a light detector, collectively referred tohereinbelow as a “LID” system. More specifically, light emanating from alight source in the injector is coupled into a first optical fiber. Thelight propagates through the first fiber and a portion of it enters asecond fiber that is to be joined to the first fiber. The lost light isdeemed transmission loss. A portion of the light in the second fiber isthen extracted and allowed to impinge on the light detector. Theextracted light is received and detected by a light responsive elementin the detector. The injection and extraction each occur at points atwhich the respective fibers are bent to a small radius of curvature. Theintensity of the light present at the light responsive element isindicative of the attenuation of light in passing from the injectionpoint to the extraction point. The attenuation is normally dominated byloss at the interface or joint between the fibers.

The present splicing system advantageously employs a LID system toeffect optimal alignment of the fibers prior to the actual fusionsplicing. The fibers are adaptively moved relative to one another toeffect an alignment, which maximizes the transmission of light acrossthe gap between the fibers prior to initiation of the fusion process.Comparison of the measured attenuation before and after fusion permitsan approximate determination of the final insertion loss of the splice.

Advantageously, the present system in its various embodiments allows thefibers to be efficiently and precisely aligned prior to fusion. Accuratealignment advantageously results in a low-loss joint, i.e., a jointthrough which a light signal may propagate with its signal strength andintegrity maintained, because the attenuation and back reflectionattributable to the joint are rendered extremely low. Preferably, jointsmade with the present apparatus have an average loss of less than about0.03 dB, and more preferably, an average of less than about 0.02 dB.Most preferably, every joint has a loss of less than about 0.02 dB.

In one aspect of the invention the LID system comprises a light injectorhaving an injector base attachable to a substrate and a light detectorhaving a detector base attachable to a substrate. The light injectorpreferably comprises (i) an opaque injector cover, at least a portion ofwhich is slidably movable in a plane parallel to the injector base, themovable portion having an open position and a closed position, the openposition permitting insertion of the first fiber into the injector; (ii)an injector window having a substantially planar entry face and aconcave, arcuate exit face; (iii) an injector mandrel having a shapecomplementary to that of the exit face of the injector window, and beingbiased to clasp a portion of the first optical fiber in intimate contactbetween the injector mandrel and the exit face of the injector window,the injector mandrel being reversibly retractable from the exit face inresponse to motion of the injector cover from the closed position to theopen position thereof; (iv) a light source positioned proximate theentry face of the injector window; and (v) optionally, a focusing lensnear the exit face of the injector window onto which the fiber ispressed by the mandrel, whereby light emanating from the source passesthrough the injector window into the first buffer and thereafter intothe first fiber core. The first fiber enters the injector in an entrydirection and emerges from the injector in an exit direction, the entryand exit directions being substantially parallel. The first fibertraverses a path through the injector substantially in a plane, which issubstantially parallel to the injector base.

The LID system further employs a light detector preferably comprising:(i) an opaque detector cover, at least a portion of which is slidablymovable in a plane parallel to the detector base, the movable portionhaving an open position and a closed position, the open positionpermitting insertion of the second fiber into the detector; (ii) adetector window having a concave, arcuate entry face and a substantiallyplanar exit face; (iii) a detector mandrel having a shape complementaryto that of the entry face of the detector window, the detector mandrelbeing biased to clasp a portion of the second optical fiber in intimatecontact between the detector mandrel and the entry face of the detectorwindow, and the detector mandrel being reversibly retractable from theentry face in response to motion of the detector cover from the closedposition to the open position thereof; (iv) a light responsive elementto detect light emerging from the fiber, the light responsive elementbeing positioned proximate the exit face, whereby light emanating fromthe buffer passes through the detector window into the light responsiveelement; and (v) optionally, an optical filter interposed between thedetector window and the light responsive element. The second fiberenters the detector in an entry direction and emerges from the detectorin an exit direction, the entry and exit directions being substantiallyparallel. The second fiber traverses a path through the detector in aplane, which is substantially parallel to the detector base.

High pass, low pass, and bandpass filter materials suitable for theoptical filter used in the present LID detector are known in the opticalmaterials art. Preferably an optical filter is selected thatpreferentially transmits light of the wavelength emitted by a LIDinjector associated with the LID detector but which strongly attenuatesor blocks both extraneous ambient light and light of other wavelengthscarried by the optical fiber. Use of such a filter material beneficiallyenhances the signal to noise ratio of the LID detection system.

The light source in the light injector is operably connected to, andenergized by, a driver. A receiver determines the intensity of lightincident on the light responsive element of the light detector. The LIDsystem, along with the other components of the splicing system, isoperable in any spatial orientation, facilitating the splicing system'suse in awkward locations.

Preferably, the LID injector and detector are attached to a commonsubstrate and oriented such that the supply ends of the first and secondfibers enter the injector and detector, respectively, in directions thatare substantially collinear. Likewise, the free ends of the fibers to bejoined emerge from the injector and detector, respectively, along acommon direction that is generally parallel the aforementioned supplydirection and only slightly displaced therefrom. This disposition of theLID components allows the fibers to be inserted in the LID systemdespite the availability of only a minimum amount of free slack. As aresult, the system is operable in close proximity to a wall, cable,conduit, or other location where fiber is present. By way of contrast,prior art systems have required much larger fiber loops withcorrespondingly more slack required and so were frequently not operablein tight quarters.

A number of structural and operational advantages are provided by theconfiguration of the present fiber splicing system and method. Thearrangement of the fiber in the system is simple and direct, the pathremaining substantially in a single plane parallel to the substrate onwhich the injector and detector are situated. Moreover, the pathdeviates from a straight line only insofar as necessary to providesufficient bending to allow insertion and extraction of light foroperation of the LID technique. As a result of this simple configurationand component design, the present LID-based fusion splicing system beoperated in very restricted quarters, such as very close to a wall,ceiling, floor, or cable support structure such as a cable tray, and incircumstances wherein access and the amount of slack available forinsertion of fibers into the system are strictly limited.

The present system is also simple to operate. In an aspect of theinvention, the mounting of the fiber is accomplished simply. The fibersare easily inserted in the injector and detector devices, as each ispreferably fully actuated by manipulation of its respective cover.Opening the cover retracts the corresponding mandrel, allowing the fiberto be inserted and properly situated; closure secures the fiber in itsoperational position. The preparation and dimensioning of the fibers isfacilitated by using an offline preparation apparatus in which a fiber,premounted in the splicer's removable holding means, is temporarilyplaced. The simplicity of these operations allows them to beaccomplished by workers who lack extensive training. Moreover, fiber canbe mounted with a minimum of manual dexterity and manipulation, as wellas in adverse conditions, such as bad weather or poor lighting, whichmake it difficult or impossible for the operator to see the equipmentand the workpieces.

The utility of the present LID system is further enhanced in someembodiments by use of short wavelength light for the LID source, e.g.light having wavelength of about 850 nm, instead of the 1310 nm or otherwavelengths typically used in previous alignment systems for fusionsplicing. The shorter wavelength is advantageous for several reasons.Available light sources operating at 850 nm are brighter and cost less.The 850 nm light is less attenuated by typical buffer coatings. Inaddition, interference in the LID detector from signals at 1310 or 1550nm present in actively operating fibers is markedly reduced. In manycases these advantages also allow the LID system to be operated withoutthe need for coupling gels previously required. The use of gels isinconvenient and further complicates the splicing process. Moreover, themode field diameter of a fiber is slightly smaller at shorterwavelengths, improving the achievable precision of active core-to-corealignment.

The LID system used in the present fiber optic splicing system enables abetter precision of alignment than attainable with other known joiningapparatus, including those employing known profile alignment (PAS)systems. PAS systems are inherently diffraction-limited, and so cannotbe made more precise than about the wavelength of the illuminatinglight, which is normally in the visible range of about 400-700 nm. Thislimitation, along with the pixel resolution of available electro-opticdetector systems (typically of the order of 1 μm) poses a substantialproblem when attempting to align small diameter, single mode fibers thatare quite commonly used in advanced long-distance data transmissionsystems. By way of contrast, the present LID system is not so limited.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood and further advantages willbecome apparent when reference is had to the following detaileddescription of the various embodiments of the invention and theaccompanying drawings, wherein like reference numerals denote similarelements throughout the several views, and in which:

FIG. 1 is a top plan view schematically depicting a portion of a fusionsplicing system incorporating one embodiment of the local injection anddetection system of the invention;

FIG. 2 a is a partially cutaway, plan view depicting the top side of alight injector used in the present system;

FIG. 2 b is a partially cutaway, plan view depicting the underside ofthe light injector shown by FIG. 2 a;

FIG. 3 a is a partially cutaway, plan view depicting the top side of alight detector used in the present system;

FIG. 3 b is a partially cutaway, plan view depicting the underside ofthe light detector also shown by FIG. 3 a;

FIG. 4 depicts in top plan view a fusion splicing stage used in thepresent system and incorporating electric arc fusion electrodes, a fiberclamping and micropositioning system, and an optical system forvisualizing fibers being spliced;

FIG. 5 depicts portions of the motion means and holding means also shownin FIG. 4;

FIG. 6 a depicts in side elevation view a piezoelectric actuator that isused in the motion means in one embodiment of the invention to providetwo orthogonal transverse motions for the fiber being spliced;

FIG. 6 b depicts in bottom plan view the actuator also seen in FIG. 6 a;

FIG. 7 depicts in side elevation view a piezoelectric actuator that isused in the motion means in one embodiment of the invention to provideaxial motion for the fiber being spliced; and

FIG. 8 depicts in cross-section view at level VIII-VIII the splicingstage also included in the embodiment of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a compact, low profile apparatusand system for joining optical fiber waveguides to produce a durablefusion splice between a first and a second optical fiber. The joinedfiber advantageously exhibits low attenuation. Advantageously thepresent system employs an adaptive technique to optimize the alignmentof the fibers prior to fusion, whereby the insertion loss of the spliceis minimized.

Referring now to FIG. 1 there is depicted generally an implementation ofa modular, low profile fusion splicing system 10 of the invention.Fusion splicing head 1 of the invention incorporates a local injectionand detection system. First and second optical fibers 20, 30 arepositioned in light injector 100 and light detector 200, aspects ofwhich are depicted in greater detail by FIGS. 2A-2B and 3A-3B,respectively. The free ends of the fibers 20, 30 appointed to be joinedare situated in facing, collinearly aligned relationship in fusionsplicing stage 300, which is further depicted by FIGS. 4-5. Activationof an electric arc between front electrode 6 and rear electrode 8 causeslocal softening of each end of fibers 20, 30, allowing the ends to bewelded, forming joint 16. Elements of fusion splicing head 1 are mountedin housing 12, which has hooks 14 for hanging head 1, e.g. in aconvenient location for carrying out field service operations. It willbe appreciated that support means other than hooks 14 may also beemployed, such as VELCRO™ attachment, brackets, support stands, and thelike. The supply ends of the fibers 20, 30 enter injector 100 and emergefrom detector 200 in substantially collinear directions. The design ofsplicing head 1, including both the configuration of injector 100,fusion stage 300, and detector 200, and the path of the fibers 20, 30through the system, is simple, allowing a splicing system comprisinghead 1 to be used for joining fibers in close proximity to walls,ceilings, cable support structures, and the like. This functionality isenhanced by a compact and low profile design for splicing head 1. By“low profile design” is meant a design wherein the extent of thesplicing head in a vertical direction perpendicular to the plane of thehead is small and not substantially increased by opening and closinginjector 100, detector 200, and the clamping means of fusion stage 300.

The embodiment of the splicing system depicted by FIG. 1 comprisesadditional modules housing a user interface 40, a control unit 60, and ahigh voltage supply 80, all powered by a power source such asrechargeable battery pack 66. In one aspect of the invention, thesemodules are housed in separate enclosures, allowing the major componentsof the system except for fusion head 1 to be mounted on a belt or vestconveniently worn by a system operator. Locating components of thesplicing system that are not required to be proximate the actual fibersin other housings advantageously permits the fusion head 1 to be madesmall in size. As a result, the present system is highly suited forfield service splicing applications that often arise in restrictedquarters. Operation and automatic sequencing of the splicing operationsare carried out through control module 60, which preferably comprises amicroprocessor and electronics associated with the operation of the LIDsystem and analysis of data provided by one or more cameras in head 1.In the depicted embodiment rechargeable battery pack 66 connected tocontrol unit 60 by cable 68 provides power needed for all the controland operational functions of system 10. However, the unit may also bepowered by any other source of electrical energy, such as energy fromthe conventional 120/240 VAC mains or a vehicle-mounted or freestandinggenerating system or battery pack of known form for field serviceapplications. Control signals and data are exchanged by control unit 60and head 1 through cable 54. Interface 40 includes a versatile visibledisplay 48, on which may be depicted at various times a menu of useroptions, a report of data collected in the course of a splicingoperation, setup and calibration information, and a magnified display ofone or more views of the fibers in the vicinity of splicing stage 300.The user enters commands in a familiar way to operate the splicer. Insome embodiments, each button is dedicated and labeled with acorresponding function, which might be “CLEAN,” “UP,” “DOWN,” “ENTER,”“EXIT”, “ON/OFF,” or other similar functions. In other embodiments, thedisplay is used to provide a context-sensitive, hierarchical choice ofmenu commands which are activated using familiar touch screenfunctionality. High voltage supply 80 provides the potential needed tostrike an electric arc between electrodes 6 and 8 of splicing stage 300.Interface 40 may communicate bidirectionally with splicing head 1through cable 44 and with control unit 60 through cable 42. One ofordinary skill in the relevant art will recognize that in otherembodiments the components needed to carry out the various functionsassociated with user interface 40, control unit 60, high voltage supply80, and battery pack 66 may be disposed in more or fewer housings thanare shown by FIG. 1. Moreover, the wiring required for the operableinterconnection of the system's components may be provided using avariety of cabling arrangements. For example, individual cables mayconnect splicing head 1 with interface 40 and high voltage supply 80,respectively, as shown by FIG. 1. Alternatively, these connections maybe shared in a single cable, such as a single cable between control unit60 and splicing head 1. As an alternative to conductive wiring, opticalfiber or wireless interconnection is optionally used for some or all ofthe control and data functions.

Other embodiments of the system incorporate further data andcommunication elements, such as storage of a log memorializing routinecalibrations, splicing events and data associated therewith, such asinsertion loss and positioning information, and images of the splicedfiber. Such information is optionally printed by a printer associatedwith user interface 40 in a conventional manner or uploadable to acomputer by wired or wireless interface protocols known in the art or bystorage in a writeable data storage means. Such storage may beimplemented using any form of semiconductor, magnetic, or ferroelectriccomputer memory or the like or by a removable mass storage medium suchas a magnetic or optical disk, flash memory modules, or other knownremovable semiconductor, magnetic, or ferroelectric memory modules.

Fusion splicing head 1, which is depicted in greater detail by FIG. 4,includes light injector 100, light detector 200, and fusion splicingstage 300. A first fiber is held by removable first clamp assembly 600a, which includes flat portion 540 a, aligning V-block 542 a, and firstfiber clamp 544 a openable at pivot 545 a. The vertex of V-block 542 aand the flat surfaces of flat portion 540 a and clamp 544 a arecoplanar. A second fiber is held by removable second clamp assembly 600b, including flat portion 540 b, aligning V-block 542 b, and secondfiber clamp 544 b openable at pivot 545 b. Second clamp assembly 600 bis generally a mirror image of assembly 600 a.

In carrying out a fusion splicing operation, first clamp assembly 600 ais preferably removed to a convenient location and first fiber clamp 544a is opened by rotating it about pivot 545 a at a side lateral to thefiber path defined, e.g., by the V-groove of block 542 a. The firstfiber is then secured by closing first clamp 544 a. Flat portion 540 aprovides a suitable place for the operator to place his/her thumb totemporarily stabilize the fiber during this operation. The fiber isthereafter prepared by removing a requisite portion of the buffer andcladding, if present, and cleaving the fiber to provide a mating surfacethat is clean, flat, and perpendicular to the fiber axis, and thussuitable for fusion joining. Preferably the underside of first clampassembly 600 a is provided with fiducial alignment pins permitting it tobe reproducibly located in both splicing head 1 and in an auxiliarypreparation apparatus used for the aforementioned buffer and claddingremoval and fiber cleaving. Advantageously such a preparation apparatusallows the axial extent of buffer/cladding removal and the length offiber projecting from the fiber clamp 544 a end of clamp assembly 600 ato the mating surface to be established reproducibly. After fiberpreparation, first clamp assembly 600 a bearing the first fiber isreplaced in head 1. A similar operation is preferably carried out tomount and prepare second fiber 30 in second clamp assembly 600 b.

The use of an auxiliary, offline fiber preparation and mountingapparatus in conjunction with the present splicing system is especiallyadvantageous for field operations, since the fiber ends can be preparedand dimensioned and accurately placed in the splicer under conditions inwhich limited visibility, difficult working conditions, or insufficientclearance hamper the dexterity of a splicing technician. By way ofcontrast, previous splicing systems typically have relied on the skillof the technician in preparing the configuration of the fiber to bejoined and in placing it accurately in the system.

After both fibers have been prepared and the respective clamp assembliesholding them have been replaced in splicer head 1, the distal ends ofthe fibers that emerge from the flat portion end of the clamp assembly,e.g. from the flat portion 540 a end of system 600 a, are mounted ininjector 100 and detector 200.

Preferably the free ends of both fibers are further secured as close tothe joint location as possible to prevent vibration and movement aftercompletion of alignment and during the welding process. In theembodiment of FIG. 4, the two fibers are respectively secured using flatportions 546 a and 546 b of movable joint clamps. The flat portions 546a and 546 b mate with complementary end portions of positioners 500 a,500 b, respectively, to create a mild pinching action capturing thefibers. The ends of the positioners are provided with precisionV-grooves 565 a, 565 b that precisely and reproducibly locate respectivefibers 20, 30. Each of the movable joint clamps pivotally attaches toits corresponding positioner 500 a, 500 b at pivot mounts 563 a, 563 b,respectively. Opposite flat portions 546 a, 546 b, on the joint clampsare respective bifurcated portions 549 a, 549 b, which engage rotatablesleeves 558 a and 558 b mounted on one end of actuator linkages 514 a,514 b. In the depiction of FIG. 4, for illustrative purposes linkage 514a and flat portion 546 a are shown in retracted, open position, whilelinkage 514 b and flat portion 546 b are shown in closed position. Inaddition a slidably movable fusion cover of fusion stage 300 has beenremoved for clarity. It will be understood that in normal operation,attachments 516 a and 516 b at the ends of linkages 514 a and 514 bopposite sleeves 558 a and 558 b are attached to the same movable fusioncover. Moving the cover operates both linkages 514 a, 514 b and bothflat portions 546 a, 546 b conjunctively. When the cover is in therearward, open position, access for inserting the fiber into the fusionstage and its joint clamps is provided; in the cover's closed position,the joint clamps are closed to secure the fibers preparatory to fusion.Preferably the cover is opaque to reduce light interference with theoperation of the fiber imaging optical system.

Fusion head 1 further comprises mechanical motion means for activelyaligning the fibers prior to fusion. In the embodiment of FIG. 4, thevertices of V-blocks 542 a, 542 b passively establish approximate fiberalignment in both the x and y directions, which are transverse to thecommon fiber axis. Precision V-grooves 565 a and 565 b further passivelylocate and secure the fibers at a point close to the splicing location.Active positioning is further carried out in both transverse (x,y) andaxial (z) directions using a combination of motor and piezoelectricdrives. More specifically, rough axial positioning is effected byindependently operable stepper motors 502 a, 502 b. The motors arecoupled by couplings 504 a, 504 b to lead screws 506 a, 506 b, which inturn rotate to drive carriages 520 a and 520 b on which are mountedpiezoelectric manipulators 500 a, 500 b. Carriages 520 a and 520 b rideon slide bars 531 a and 531 b with interposed bearings 530 a and 530 b,respectively, to provide smooth, low friction travel. Suitable actuationof motors 502 a and 502 b thus permits the fibers, which are secured byfiber clamps 600 a, 600 b to piezoelectric manipulators 500 a, 500 b, tobe moved axially into approximate abutment. Independent motion of thetwo motors permits the point of abutment to be located symmetricallyalong an imaginary line connecting electrodes 6, 8, for optimal arcwelding. Direct drive of carriages 520 a, 520 b advantageouslyeliminates the backlash and other similar imprecision that normallyattends gear-driven motion systems such as those frequently employed inprior art splicers. In addition, a direct drive system is more compact,simpler, and far less prone to breakdown. Together, these factorscontribute to the ruggedness and portability of the present system.

More precise alignment of the fibers is preferably carried out usingpiezoelectric drives 500 a and 500 b, best visualized by reference toFIG. 5. In the embodiment shown, drive 500 a provides, y-axis transversemotion and z-axis axial motion, while drive 500 b provides transverse,x-axis motion. While one skilled in the art will recognize that analysisand alignment is simplest in a system with drive capabilities in threeorthogonal directions, it will also be appreciated that any systemcapable of providing sufficient motion in three non-collinear directionscan bring fibers into proper alignment. Other system embodimentsoptionally permit each of the fibers to be moved independently in threedimensions, but apportioning three directions over two positioners, onefor each fiber, as in the embodiment of FIGS. 5-7, will be recognized assufficient for fiber alignment. The analysis is facilitated by selectingimaging directions that are transverse to the fiber axis and mutuallyorthogonal. However, known mathematical transformations can be used toprovide the requisite positioning information as long as the imagingdirections are not coincident. The alignment process is furtherfacilitated by selection of positioning motions that are also transverseand mutually orthogonal.

Referring now to FIGS. 6-7 the operation of piezoelectric drives 500 aand 500 b in one embodiment may be visualized. FIGS. 6 a and 6 b showright piezoelectric drive 500 b that provides transverse actuation offiber 30 in two orthogonal transverse directions. Fiber 30 (not shown)is secured between fiber clamp 561 b and fiber guide 568 b and coupledto the active elements by piezoelectric mount 570 b and interstage clamp580 b in housing 578 b. First and second y-direction piezoelectricbi-morphs 574 b and 576 b form a couple to deform to produce y-axismotion, while the couple of piezoelectric bi-morphs 572 b and 573 bprovide x-axis motion. FIG. 7 illustrates left piezoelectric drive 500 ain which fiber 20 is mounted between fiber guide 568 a and a fiber clamp(not shown). Piezoelectric bi-morphs 575 b and 577 b provide axiallydirected (z) motion.

While the stepper motors and piezoelectric actuators depicted in FIGS.5-7 are presently preferred for the motion means of fusion stage 300,other forms of pneumatic and electromechanical actuators capable ofproducing the requisite extent of linear or rotary motion may also beused in practicing the present invention.

The optical imaging system of stage 300 preferably comprises a lightsource and detector for acquiring images of the joint area from twonon-collinear directions. Preferably, the directions are mutuallysubstantially orthogonal and perpendicular to the fiber axis. As bestvisualized in the cross-sectional view of FIG. 8, a first optical pathis defined by emission of back light from front source 302 ofbacklighting, mounted in front lamp housing 303, which illuminates thearea of joint 16. Light then passes through rear lens 324 mounted in arear lens holder 326, and successively reflects from rear fold mirror328 and refracts through rear prism 329 before downwardly impinging theactive area of camera 314. A second optical path is defined by emissionof light from rear source 320, mounted in rear lamp housing 321, whichilluminates the area of joint 16, and subsequently passes through frontlens 306 mounted in a front lens holder 308, and successively reflectsfrom front fold mirror 310 and refracts through front prism 313 beforedownwardly impinging on camera 314. The images captured by camera 314are magnified by the lenses and optical system design, preferably atleast about 10×, and more preferably, at least about 20×. Preferably, aplurality of tilt adjustment screws 319 secures each of mirrors 310 and328, whereby the optical system can be brought into alignment andsecured so that images of the optical fibers in two substantiallyorthogonal directions are captured in different portions of the activearea of camera 314. The first and second optical paths preferably lie ina plane normal to the common fiber axis and passing through the point ofabutment of the fibers.

In the optical system depicted by FIG. 8, some of the elements in eachoptical path are located above the fiber plane, and some are below. Inaddition, each optical path is multiply folded. That is to say, the pathincludes plural, non-collinear segments, that multiply change directionas a result of the reflective or refractive elements within the path.More specifically, sources 302 and 320 are located above the fiberplane, i.e., a horizontal plane traversed by the fibers mounted in theLID detector and injector and splicing stage. The remaining opticalcomponents, including lenses, mirrors, prisms, and reflectors, andcamera, are below the fiber plane. The optical system depicted by FIG. 8is advantageously compact as a consequence of its multiply-foldedoptical paths which: (i) lie in a plane perpendicular to the commonfiber axis at the location of the fusion joint and (ii) penetrate thefiber plane. Advantageously the plane of the optical paths also containsthe electrodes 6, 8 of the welding system, minimizing parallaxdistortion of the fibers near the ultimate joint location. In addition,the optical system employs a single camera 314, which may be anysuitable electro-optical image detector having the requisite size,sensitivity, and resolution, but is preferably a charge-coupled or CMOSdevice. Images of the fiber in substantially orthogonal directions areprojected onto different sections of the camera's sensitive area. Thecamera is connected to suitable analog or digital electronic processingcircuitry that produces an image that may be displayed in real time ondisplay 48 in the user interface unit 40. The processing optionallyincludes image enhancement and processing using known image improvementtechniques.

The acquired fiber images are optionally used as input to a PAS systemwhich carries out an adjustment of fiber positioning. Known electronicimage analysis techniques, preferably implemented using a microprocessoror comparable circuitry in control unit 60, are used to ascertain thecondition and relative orientation and position of the respective endsof the two fibers. The circuitry then adaptively commands thepositioning system in head 1 to move the fibers into approximate coaxialalignment and abutment. The PAS system preferably operates iterativelyto bring the fibers into as accurate alignment as the diffraction limitand resolution of the imaging optics permit. After PAS alignment iscompleted, the fibers have sufficient optical coupling for a LID systemto function.

Referring now to FIGS. 2A-2B there is depicted one form of a low profileLID injector 100 for injecting light into an optical fiber waveguide 20.Injector 100 is mountable on a substrate and is covered by a fixed coverportion 101 and a slidably movable cover portion 102. Movable portion102 has peripheral splines 105 on each side as partially shown by FIG.2A which engage complementary slots 109 in fixed portion 101 to maintainlinear alignment of the portions 101, 102 during the motion of movablecover portion 102. As best visualized by reference to FIG. 2B, openingmovable cover 102 of injector 100 retracts injector mandrel 112.Retraction of mandrel 112 also allows access to fiber path 150, asdepicted by FIG. 2A. Closing cover 102 returns mandrel 112 to bear onfiber 20, which is thereby grasped between arcuate, concave surface 153of injector window 154 and the upper portion of mandrel 112. Cover 102actuates mandrel 112 through the action of a mechanical linkagecomprising crank actuator 104 and crank 106. One end of crank actuator104 is attached by a screw 103 to a threaded boss on the underside ofcover 102. The other end of crank actuator 104 is rotatably attached bypin 108 to clevis 107 at one end of crank 106. Crank 106 is pivotallyattached at a point intermediate mandrel 112 and pin 108 by a screw 110to a boss on the underside of optics mount 116. Mandrel 112 is disposedin a hole at the end of crank 106 opposite clevis 107. The opening andclosing of cover 102 thereby moves mandrel 112 through mandrel guideslot 114 in mount 116. Mandrel 112 is preferably composed of aferromagnetic material, such as a magnetic stainless steel. When cover102 is in the closed position, the lower portion of mandrel 112 isproximate one or more permanent magnets. Mandrel 112 acts to close themagnetic circuit formed in cooperation with the magnets. The resultingattractive force acting on mandrel 112 is communicated through crankactuator 104 and crank 106 to urge cover 102 into closed position.

In the closed position, movable cover portion 102 and fixed coverportion 101 cooperate to shield the components of LID injector 100 fromexternally incident light. However, light generated by injector lightsource 152, which is electrically energized through leads 130, isfocused by a lens and then enters and passes through entry surface 156of injector window 154, emerges through concave surface 153 of window154, and enters fiber 20 through the buffer coating thereof in theportion of the fiber bent and held in conformity to surface 153 bymandrel 112. Preferably concave surface 153 and mandrel 112 havecomplementary shape. Disposition of fiber 20 in clasping contact betweenarcuate surface 153 and mandrel 112 deflects fiber 20 sufficiently forlight from source 152 incident on the buffer jacket of fiber 20 to beinjected into the fiber core for propagation therethrough. Preferablythe focusing lens is in the form of a right circular cylinder of glasshaving a radially graded refractive index and is disposed with itscylindrical axis substantially coincident with the optical path. Oneform of such lenses is sold commercially by Nippon Sheet Glass under thetradename “SELFOC.” However, lenses of other known types, includingconventional convex lenses composed of conventional optical materialsmay also be used in constructing the optical system of the presentinjector.

Light source 152 may comprise any means of illumination but preferablycomprises a light emitting diode (LED). A semiconductor laser or othersuitable light source may also be used. The use of a source that emitsat a short wavelength (e.g. a wavelength ranging from about 800 to 900nm, and preferably about 850 nm) is preferred, as discussed in greaterdetail hereinabove.

FIGS. 3A-3B depict a LID detector 200 of the invention. LID detector 200corresponds to LID injector 100 depicted by FIGS. 2 a-2 b and is similarin structure, mechanical operation, and engagement of an optical fiberpassing through it. A light responsive detection element replaces lightsource 152 of injector 100. Preferably detector 200 has a structurewhich is generally a complementary, mirror image of injector 100 so thata LID system comprising both has improved compactness.

More specifically, in FIGS. 3A-3B there is depicted one form of a lowprofile LID detector 200 for injecting light into an optical fiberwaveguide 30. Detector 200 is mountable on a substrate and is covered bya fixed cover portion and a slidably movable cover portion. Peripheralsplines on each side of the movable cover engage complementary slots toassure that the cover portions remain aligned during actuation of themovable cover portion. Opening the movable cover of detector 200retracts detector mandrel 212, allowing access for placing fiber 30 intofiber path 250. Closing the movable cover returns mandrel 212 to bear onfiber 30, which is thereby grasped between arcuate, concave surface 253of detector window 254 and the upper portion of mandrel 212. The movablecover actuates mandrel 212 through the action of a mechanical linkagecomprising crank actuator 204 and crank 206. One end of crank actuator204 is attached by a screw 203 to a threaded boss on the underside ofthe movable cover. The other end of crank actuator 204 is rotatablyattached by pin 208 to clevis 207 at one end of crank 206. Crank 206 ispivotally attached at a point intermediate mandrel 212 and pin 208 by ascrew 210 to a boss on the underside of detector optics mount 216.Mandrel 212 is disposed in a hole at the end of crank 206 oppositeclevis 207. The opening and closing of the movable cover thereby movesmandrel 212 through detector mandrel guide slot 214 in mount 216.Mandrel 212 is preferably composed of a ferromagnetic material such as amagnetic stainless steel. When the cover is in the closed position, thelower portion of mandrel 212 is proximate permanent magnets which aredisposed in blind holes in detector optics mount 216. Mandrel 212 actsto close the magnetic circuit formed in cooperation with the magnets.The resulting attractive force acting on mandrel 212 is communicatedthrough crank actuator 204 and crank 206 to urge the movable cover intoclosed position.

In the closed position, the movable and fixed cover portions cooperateto shield the components of LID detector 200 from externally incidentlight. However, a portion of this light propagating through fiber 30 isextracted therefrom at a bend in fiber 30 at the location where fiber 30is clasped between mandrel 212 and concave surface 253. The extractedlight passes through the buffer of fiber 30 and enters window 254through entry surface 253. Light emerges from window 254 through exitsurface 255.

A light responsive element abutting exit surface 255 of window 254 maycomprise any electronic element whose electrical characteristics changein response to the incidence of light thereon. Preferably the lightresponsive element comprises a phototransistor, Si or InGaAs PIN diode,avalanche photodiode (APD), or other element electrically responsive tolight of the wavelength emitted by light source 152. A Si PIN diode ispreferred for its availability, low cost, low noise, and immunity toradiation of wavelength longer than about 1050 nm.

It is also preferred that a filter that substantially transmits thelight from source 152 but excludes other wavelengths be interposedbetween surface 255 and the light responsive element.

Embodiments employing 850 nm LID are advantageously employed in splicingfiber systems appointed to transmit data using longer wavelength light,e.g., 1310 or 1550 nm light. Properly chosen filters then exclude thedata light from the LID detector. In such systems, a Si PIN diode LIDdetector is preferred as being strongly responsive to 850 nm LID lightbut not to 1310 or 1550 nm light.

LID injector 100 depicted by FIGS. 2A-2B and LID detector 200 depictedby FIGS. 3A-3B are advantageously used in a compact, low profile,modular fiber optic splicing system. Each of injector 100 and detector200 is operated by sliding its respective cover to the open and closedpositions. The simplicity of these operations allows optical fiber to beeasily placed within the unit and subsequently removed after thesplicing operation is completed. The LID system is employed by thesplicing system to achieve proper of the alignment of the fibers so theymay be joined in a joint that exhibits minimal insertion loss. Theaforementioned LID injector and detector system advantageouslycontributes to the design and operation of a fusion splicing system thatis thereby compact, portable, and easily operated even under adverseenvironmental conditions and in cramped quarters. Additional forms oflow profile LID systems are disclosed in copending application bearingAttorney Docket No.: 0040-7, which is filed of even date herewith,commonly assigned, and incorporated herein in the entirety by referencethereto.

The LID system provides a signal indicative of the intensity of light inthe second fiber that has been injected by LID injector 100 into thefirst fiber and propagated across the interface therebetween. A lightresponsive element in LID detector 200 senses the propagated light. Theoutput of the element is fed to suitable electronic circuitryincorporating amplification and filtering to produce a measured signalused as feedback to drive a servo system to bring the fibers into finealignment in three dimensions. Optimized alignment, which is signaled bya maximum in the transmitted light intensity, is essential in forming adurable fusion splice with minimal or no insertion loss. The LID systemadditionally provides a method for inferring the actual insertion lossof the spliced fiber by comparison of the transmission between thefibers before and after splicing. The theoretical loss due to theinterface between two fibers having index of refraction of about 1.4 isestimated to be about 0.36 dB. Thus, the increase in transmission aftersplicing is decremented by 0.36 dB to provide an inferred insertion lossof the splice.

Fusion splicing stage 300 further comprises an electric arc weldingsystem for fiber joining. Preferably the system employs electrodes 6, 8mounted in horizontal, transverse, axially opposed relationship asdepicted by FIG. 1. In addition, it is preferred that the electrodes belocated in the same vertical plane as the components of the imagingoptical system. Electrodes 6, 8 are energized by high voltage supply,triggered automatically by control electronics after completion of finefiber alignment.

A suitable arc softens and welds the fiber ends to form a durable, lowloss splice. Known electrical supply means are used to drive the arc ina reliable manner, the electrical characteristics thereof beingpreselected through the user interface. Too intense an arc melts thefibers excessively, causing formation of a ball-like end that retreatsfrom the joint area. Too weak an arc does not allow enough heating tocause a mechanically stable joint to form.

In another aspect of the invention, there is provided a method of fusionsplicing optical fibers using system 10. Fibers 20 and 30 appointed forjoining are mounted by an operator in LID injector 100 and detector 200.The ends of fibers 20, 30 are further secured in fusion stage 300 foralignment and splicing.

The fusion operation is initiated by preparing the fibers, preferably byremoving the buffer and cladding layers, if any, from the fiber, andalso cleaving the ends of the fibers to provide a joining surface at theend of each that is substantially planar and perpendicular to the fiberaxis. The respective fibers are then placed in the clamp assemblies ofstage 300. These clamp assemblies preferably comprise precisionV-blocks, of a form typically used in machining operations, with clampsto hold the fibers securely therein. Even though the V-blocks hold therespective fibers in approximate collinear alignment, the accuracy ofthe axial separation and lateral positioning after initial mounting areinadequate for fusion joining. Therefore, the splicing stage 300 ispreferably provided with further electronically controlled motion meansfor adaptively bringing the fibers into alignment that is sufficientlyprecise to produce a low transmission loss splice.

Preferably the adaptive alignment is carried out in an automatic cycleinitiated by an operator, such as by depressing appropriate buttons 46of interface 40. After the fiber ends are brought into optimal alignmentby the positioning system in head 1, a firing sequence initiates anelectric arc of the requisite intensity and duration to fusion join thenow-contiguous ends of fibers 20 and 30.

The first stage of aligning the fibers may be carried out manually,preferably with the assistance of images of the respective fibers takenin two mutually perpendicular optical directions normal to the commonfiber axis. The images are conveniently acquired using the opticalsystem in splicing stage 300 and the electronics associated therewithand presented on display 48. More preferably, the alignment comprisesuse of an automated PAS system to carry out an initial three-dimensionalalignment. The PAS system employs electronic processing of the fiberimages to spatially locate the fibers and quantitatively determine theirmisalignment. The positioning system in splicing stage 300 is thenactuated to bring the fibers into alignment. The process may be carriedout iteratively until the alignment is within the measurement toleranceand resolution of the PAS optical system.

Use of an automated PAS system under system control for the initialalignment is especially preferred in field repair or installationsituations wherein environmental or working conditions impede manualoperations. In particular, PAS alignment can be effected even in caseswhere the fibers are initially mounted so far out of alignment thatlight injected by a LID injector does not traverse the inter-fiber gap,precluding any adaptive optimization solely using the LID system.However, as previously noted, the alignment accuracy attainable with PASis diffraction limited, thereby also limiting the typically attainabletransmission loss in joined fibers.

To overcome the inherent limits of a PAS-based splicing system, thealignment sequence in an aspect of the present method and system furtheremploys a LID system. The LID system incorporates means for injectinglight into the first fiber through its buffer layer and correspondingmeans for detecting the intensity of light emerging through the bufferlayer of a second fiber. Optimal fiber alignment prior to splicing iseffected by manipulating the orientation and relative position of thefibers to maximize light transmission. In the LID method, light incidenton the buffer jacket of the first fiber at an injection positionpenetrates the buffer and cladding, enters the core, and propagatesthrough the first fiber, gap, and second fiber, emerging from the coreof the second fiber through its cladding and buffer at a detectionposition. These processes require that the fibers be bent at theinjection and detection positions. Otherwise, light is constrained bytotal internal reflection to remain in the fiber core and solely topropagate therethrough.

A low profile LID system and a compact fusion splicing stage such asthose aforementioned are advantageously employed in the construction ofa modular, low profile system for fusion splicing of optical fibers. TheLID injector and detector are conveniently mountable on the oppositelateral sides of the fusion splicing stage and in close proximitythereto, as depicted by FIGS. 1 and 4. The LID injector and detectorboth have a low profile, having no need for clearance above the devicesto accommodate the open position of the upwardly rotatable closuresnormally used in conventional systems for mounting, securing, anddeflecting fibers. This configuration conveniently affords a paththrough the head of the splicer system for the two optical fibers beingjoined that is simple and direct. The fibers remain substantially in asingle plane parallel to the surface of the splicer head, traversing apath that deviates from a straight line only insofar as necessary toprovide sufficient bending to allow injection and extraction of lightfor operation of the LID technique. As a result, the vertical extent ofentire splicing stage is minimized, further lowering the profile of thepresent system. Preferably the LID components and the fusion splicingstage are configured as depicted in FIG. 1. The supply ends of firstfiber 20 and second fiber 30 enter injector 100 and detector 200,respectively, in directions that are substantially collinear. Likewise,the free ends of the fibers 20, 30 to be joined in joint 16 emerge frominjector 100 and detector 200, respectively, along a common directionthat is generally parallel the aforementioned supply direction and onlyslightly displaced therefrom. Furthermore, the LID injector and detector100, 200 and the fusion splicing stage 300 are preferably situated closeto an edge of the splicing head housing. As far as possible, componentsthat must be in the head are located rearward of the fiber path to allowthe fiber edge to be as close as possible to the edge of the housing forgreatest operational flexibility. In a preferred embodiment, the fusionhead 1 is at most about 24 cm wide in the fiber direction, at most about12 cm front to back, and 8 cm deep, and weighs at most about 2 kg.Advantageously such a fusion head is readily hand-carried andmanipulated into position. The width of the fusion splicing head may besubstantially reduced in embodiments that do not require LIDfunctionality by omission of injector 100 and detector 200.

The compactness and rugged portability of the present splicing systemare further enhanced by features of the splicing stage. The use ofmicropositioners such as piezoelectric and direct drive motors and theconcomitant reduction or elimination of mechanical gears subject tomisalignment and backlash enable the system to withstand the inevitablemechanical abuse, including shock, dirt, moisture, and other adversitiesthat attend transporting and operating service equipment under fieldconditions. In addition, the use of lightweight micropositioners andrelated components further reduces gravity-induced bending andmisalignment that generally have required previous systems to becalibrated and operated in a single, fixed orientation. By way ofcontrast, the present fusion head advantageously is operable in otherarbitrary orientations, greatly facilitating its use for field servicein cramped quarters.

As a result of its configuration and component design, the presentfusion splicing system is compact and low profile, rendering it operablein very restricted quarters, such as very close to a wall, ceiling,floor, or cable support structure such as a cable tray. Moreover, only aminimal amount of free slack is required to situate the fibers in thesplicer. These singular and advantageous features are a consequence offactors including the minimal clearance needed on the sides, top, andbottom of a housing for a splicing head that incorporates low profile,compact components, including the components of the LID system and thesplicing stage included in the present apparatus. Other components ofthe splicing system, including power sources, electronics, and userinterface, may be connected to the splicing head but housed separately.The head itself may thus be made quite compact for operation in confinedspaces. Preferably, the interconnecting cables are terminated in plugsand receptacles of known type to permit the components of the presentsystem to be separable and removable to facilitate such functions astransportation, storage, repair, calibration, and maintenance.

The system provides means for effecting high quality, low insertion lossfiber optic splices, for which active optical techniques are essentialfor attaining sufficiently precise alignment of the fibers inpreparation for fusion splicing. The markedly improved functionality andportability afforded by the splicer of the invention is absent fromexisting systems which cannot perform high quality, low loss splices inthe tight confines and adverse environmental and operational conditionsfor which the present system is especially adapted.

Having thus described the invention in rather full detail, it will beunderstood that such detail need not be strictly adhered to but thatvarious changes and modifications may suggest themselves to one skilledin the art, all falling within the scope of the present invention asdefined by the subjoined claims.

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 31. A method for joining a first optical fiberand a second optical fiber along a common fiber axis, the methodcomprising: a) providing a low profile fusion splicing system, thesystem comprising: (i) a low profile fusion splicing head having afusion splicing stage including a clamping and fiber position adjustmentsystem comprising holding means for holding said fibers substantially ina horizontal plane and motion means for moving said fibers in threeorthogonal dimensions into coaxial, abutting alignment; an imagingoptical system having a fiber imaging illuminator and a fiber imagedetector, said imaging optical system being adapted to acquire opticalimages of said fibers in a first imaging direction and a second imagingdirection, said imaging directions being non-coincident; and an electricarc welding system; (ii) a user interface having an output display anduser input controls for activating the splicing system; (iii) electroniccontrol circuitry having imaging electronics that receive the output ofsaid fiber image detector and produce a display signal feeding saidoutput display; and fusion control electronics operably connected toactivate said electric arc welding system and supply high voltagethereto; b) preparing said first and second optical fibers by removingcoatings present thereon and cleaving the ends of the fibers to form amating end on each fiber; c) arranging said first and second opticalfibers in said holding means with said mating ends in facingrelationship; d) imaging said fibers prior to said joining; e)positioning said optical fibers into coaxial, abutting alignment; and f)fusing said fibers by electric arc welding.
 32. A method as recited byclaim 31, wherein said arranging comprises clamping said fibers inV-blocks comprised in said holding means in said fusion head.
 33. Amethod as recited by claim 32, wherein said holding means comprises afirst and a second removable clamp assembly.
 34. A method as recited byclaim 33 wherein: said fibers are mounted in said removable clampassemblies prior to said preparing of said fibers; said preparing ofsaid fibers is carried out using an auxiliary fiber preparationapparatus; and said arranging comprises placing said removable clampassemblies bearing said fibers in said fusion splicing system.
 35. Amethod as recited by claim 31, wherein said electronic control circuitryfurther comprises a profile alignment system in communication with saidfiber image detector and said positioning comprises use of said profilealignment system to command said motion means to bring said fibers intoalignment.
 36. A method as recited by claim 31, wherein: a. said fusionsplicing head further comprises a local injection and detection systemincluding a light injector adapted to inject light into said first fiberand a light detector detecting light in said second fiber, said localinjection and detection system providing an electronic intensity signalindicative of the fraction of said injected light propagated across theinterface between said fibers; b. said electronic control circuitryfurther comprises a driver energizing said light injector andmeasurement electronics connected to said light detector receiving andprocessing said electronic intensity signal to provide a measuredintensity signal; and a servo system operative to drive said actuatorsto maximize said measured intensity signal, whereby the position of saidfibers is optimized prior to fusion thereof; c. said method furthercomprises mounting said first optical fiber in said light injector andsaid second optical fiber in said light detector; and d. saidpositioning comprises use of said servo system to command said motionmeans to bring said fibers into alignment.
 37. A method as recited byclaim 31, wherein said positioning comprises translation of said fibersin three mutually orthogonal directions.
 38. A method as recited byclaim 37, wherein one of said mutually orthogonal directions issubstantially coincident with said common fiber axis.
 39. A method asrecited by claim 37, wherein said positioning comprises translation ofsaid first fiber in a first transverse direction substantiallyorthogonal to said fiber direction and translation of said second fiberin a second transverse direction substantially orthogonal to said fiberdirection and said first transverse direction.
 40. A method as recitedby claim 31, further comprising inferring the transmission loss of saidfiber after said fusing.
 41. A method as recited by claim 31, furthercomprising encasing said fused joint in a heat shrinkable sheath.
 42. Amethod as recited by claim 36, wherein said light injected by said lightinjector has a wavelength ranging from about 800 to 900 nm.
 43. A methodas recited by claim 31, further comprising storing and transferring dataassociated with said joining.